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Arcella discoides

Galeripora discoides

iNaturalist

Meet the microscopic architect of the pond world. Arcella discoides builds a perfect golden-brown disk that serves as both a fortress and a flotation device in the hidden jungles of your backyard water features.

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Quick Identification

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Size

70–150 micrometers (0.0027–0.0059 inches) in diameter

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Colors

Transparent or light yellow when young, aging to a deep golden brown or reddish-brown

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Key Features

Circular, disk-shaped shell (test) with a smooth surface
Large central circular opening (aperture) on the underside
Internal gas vacuoles used for buoyancy control
Finger-like pseudopodia used for movement

When You’ll See Them

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Activity pattern Active day and night

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Peak hours 24 hours a day

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Season Year-round, but most active in Summer and Spring

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Diet Bacterivore and herbivore; it feeds on bacteria, small algae (diatoms), and various flagellates using its pseudopodia to engulf prey.

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Habitat Freshwater environments including ponds, lakes, peat bogs, and damp mosses.

Behavior

Arcella discoides is a fascinating 'testate amoeba,' meaning it is a single-celled organism that constructs a protective shell. Unlike many other amoebas that change shape constantly, Arcella discoides lives inside its rigid, disk-like home. It moves through its environment by extending lobopodia—blunt, finger-like projections of its cellular body—through a hole in the bottom of its shell. These pseudopods act both as 'feet' for crawling across submerged leaves and as 'arms' to snag passing food particles.

One of the most remarkable behaviors of this species is its ability to regulate its position in the water column. By producing internal gas bubbles within its shell, it can effectively change its density to float toward the surface or sink back down to the sediment. This allows it to find optimal light levels for the algae it may consume or to escape stagnant, low-oxygen environments at the bottom of a pond.

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Camera Tips

Capturing Arcella discoides requires a different approach than typical backyard wildlife. Because they are microscopic, you cannot use a standard trail camera. Instead, you will need a digital microscope or a smartphone-compatible microscope adapter with at least 100x to 400x magnification. To find them, collect a sample of pond water or, better yet, squeeze the water out of a handful of damp Sphagnum moss into a small container. This 'moss squeeze' is a goldmine for testate amoebas.

Place a single drop of your sample onto a glass slide and cover it with a coverslip. To use an AI-powered camera system, look for digital microscope heads that offer a USB or Wi-Fi feed. Position your slide so the light comes from beneath (transillumination). Since Arcella discoides are slow-moving, you can set your camera to time-lapse mode to observe their gradual movement across the slide or the extension of their transparent pseudopodia.

For the best visual results, look for older specimens. Young Arcella have clear, nearly invisible shells, while older ones accumulate iron and manganese from the water, turning a beautiful, rich amber or chocolate brown. This color provides excellent contrast against the bright background of a microscope light. Avoid using heavy dyes, as these can kill the organism; instead, use a 'hanging drop' slide technique to allow the amoeba to move naturally in 3D space for more dynamic footage.

Similar Species

Species that look similar or are commonly confused with Arcella discoides.

Arcella vulgaris

Has a much higher, dome-shaped shell compared to the flat, disk-like shell of A. discoides.

Difflugia

Constructs its shell from tiny sand grains or mineral particles rather than smooth organic secretions.

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Frequently Asked Questions

Arcella discoides is active 24 hours a day. Because it is a single-celled organism, it does not follow a circadian sleep cycle like mammals, though its movement may increase in response to light levels that attract its prey, such as algae.

The best way to attract them is to create a healthy freshwater habitat. A small pond with plenty of native aquatic plants and a layer of organic leaf litter or Sphagnum moss will naturally support a population of these amoebas.

They are scavengers and predators of the microbial world, primarily eating bacteria, single-celled algae (like diatoms), and small protozoans which they engulf using their finger-like pseudopods.

Yes, they are extremely common in suburban garden ponds, birdbaths that haven't been cleaned recently, and even damp moss growing on north-facing walls or trees.

Arcella discoides is much flatter and wider, resembling a thin plate or disk. Arcella vulgaris is more dome-shaped or hemispherical, looking like a tiny bowl or umbrella when viewed from the side.

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